1. Field of the Invention
This invention relates to a microwave circuit device obtained by forming a microwave circuit on a substrate, and more particularly to an improved shield structure for shielding an circuit from external radio noise.
2. Description of the Related Art
First, a conventional shield structure employed in, for example, a microwave amplifier for amplifying a signal having a high frequency of several GHz--ten and several GHz will be explained with reference to FIGS. 1A and 1B.
FIG. 1A is an exploded perspective view of a microwave amplifier, and FIG. 1B a cross sectional view showing part of the exploded cross section of the amplifier.
In FIGS. 1A and 1B, reference numeral 11 denotes a circuit substrate, which has an upper surface divided into two portions. Integrated circuit devices 121 and 122 for amplifying microwave (hereinafter called "IC component") are provided on central portions of the divided two surface portions, respectively.
On the upper surface of the substrate 11, the output terminal of the IC component 121 and the input terminal of the IC component 122 are connected to a wiring pattern 111 by means of solder, and are thus connected to each other in series. The input terminal of the IC component 121 and the output terminal of the IC component 122 are respectively connected to wiring patterns 112 and 113 by means of solder, which patterns are to be connected to the hot terminals of coaxial connectors 131 and 132, respectively.
The circuit substrate 11 further has ground patterns 114 and 115 formed on peripheral portions of the upper surface, and a ground conductive layer 11b formed on the overall lower surface, hereinafter referred to as a "ground layer". The upper-surface-side patterns 114 and 115 are connected to the lower-surface-side ground layer 11b by means of a through hole 11a formed in a desired portion of the substrate 11.
The ground terminal of each of the IC components 121 and 122 is connected to the ground patterns 114 and 115. Screw holes 116-119 are formed in four corners of the circuit substrate 11.
The circuit substrate 11 further has microwave components such as a resistor, etc., and wiring patterns for connecting the components. These components and patterns are not explained and shown, for simplifying the explanation of the shield structure.
A shield case for shielding the substrate 11 comprises a case main body 14, and a cover 15 for covering the upper surface of the case main body 14. The body 14 and the cover 15 are made of a conductive member (e.g. aluminum member).
The thickness of the case main body 14 increases from a predetermined portion thereof, such that it can be brought into electrical contact with the ground patterns 114 and 115 formed on peripheral portions of the substrate 11, in a state where the substrate 11 is inserted into the body 14 from the bottom and fixed to an intermediate portion thereof. Those portions of the ground patterns 114 and 115 of the substrate 11 which contact the case main body 14 are not coated with a resist for preventing corrosion.
Further, a partition plate 141 is formed integral with the case main body 14 at the center thereof, for shielding the IC components 121 and 122 from each other. A groove 142 is formed in a bottom central portion of the partition plate 141 so as to prevent the plate 141 from contacting the wiring pattern 111 on the upper surface of the substrate 11.
Through holes 143 and 144 are formed in those portions of the case main body 14 to which the coaxial connectors 131 and 132, for inserting therethrough the hot terminals of the connectors.
Peripheral surface portions of the connectors 131 and 132 serve as ground electrodes, and their central portions serve as hot electrodes. The hot electrodes are connected to pin terminals, which serve as hot terminals.
The connectors 131 and 132 are screwed to the case main body 14 without permitting their hot terminals to contact the inner surfaces of the through holes 143 and 144 (this state is not shown). Thus, peripheral portions of the connectors 131 and 132, the case main body 14, the cover 15, and the ground pattern 114 of the circuit substrate 11 are electrically connected to the lower-surface-side ground layer, and are kept at the ground potential.
The portions of the case main body 14 to which the connectors 131 and 132 are attached are made thinner than the other portions, and the hot terminals of the connectors 131 and 132 which project from the holes 143 and 144 are soldered to the patterns 112 and 113 of the substrate 11, respectively. As a result, the hot terminal of the connector 131 functions as a microwave signal input terminal, and that of the connector 132 as a microwave signal output terminal.
The substrate 11 is screwed to the case main body 14 by means of screws 151-154 from the bottom of the body 14, and the cover 15 is adhered to the upper surface of the body 14 by the use of a shield resin.
No explanations will be given of a power supply system for the circuit substrate 11, so as to simplify the explanation of the device.
Since the IC components 121 and 122 are surrounded with conductive members, they can be shielded from external radio noise.
In the case of the above-described shield structure, however, when it is actually assembled, a clearance may inevitably be formed due to a warped portion of the substrate 11 and/or uneven surface portions of the ground patterns 114 and 115, as is shown in FIG. 2. External radio noise can enter into the circuit through the clearance, thereby, for example, reducing the ratio of the level of a signal to that of noise.
Moreover, since uneven surface portions of the ground patterns 114 and 115 increase the contact resistances between them and the case main body 14, a potential difference may be caused between the substrate 11 and the shield case, thereby degrading the characteristics of the circuit.
To avoid this, insertion of a conductive rubber between the patterns 114 and 115 of the substrate 11 is employed as a conventional technique. This technique, however, may increase the contact resistances of the patterns 114 and 115 since the contact resistance of a conductive rubber is ten times higher than that of a metal. Further, the conductive rubber will have its quality degraded or cracked after long use, reducing the life of the circuit device. In particular, when the rubber is cracked, external radio noise enters into the circuit, thereby reducing the ratio of the level of a signal to that of noise.
FIG. 3 shows another shield structure. In this case, a groove 145 is formed in the surface portion of the case main body 14 which contacts the circuit substrate 11, and is filled with a shield line 16 made of an elastic conductive member (which consists, for example, of a cable made of an elastic resin member and threads made of copper or aluminum and wound on the cable). The diameter of the shield line 16 is set greater than the depth of the groove 145.
In the above structure, when the substrate 11 is screwed to the case main body 14, the shield line 16 is pressed against the ground patterns 114 and 115 of the substrate 11 and the base main body 14, thereby filling the space between the patterns 114 and 115 and the base main body 14.
However, since the groove 145 to be filled with the shield line 16 is formed in the shield structure such that it does not pass the screw holes, the shield structure must be made thick. This being so, the shield case must be made large and heavy.
In addition, the shield line 16 cannot pass that portion of the circuit substrate 11 at which the wiring patterns 112 and 113 are formed. Therefore, the line 16 must be divided into two portions such that it does not contact the patterns 112 and 113. As a result, not only the man-hour of assembly is increased, but also there is a possibility of radio noise entering into the circuit from a portion at which the shield line 16 is not provided.